Pumps for transporting fluids from one point to another against a back pressure are well known, with some designs dating back hundreds or even thousands of years. The animal heart, with its muscle-driven, responsive, variable volume pumping chambers and integral non-return valves, represents a beautiful example of a pump created by nature.
In recent years there has been growing interest in the development of so-called micro pumps for pumping fluids. In general, this class of pump is physically compact, with dimensions ranging from a few millimeters to tens of millimeters, and having the ability to pump fluids at volume flow rates ranging from fractions of a milliliter up to a several milliliters per minute. The interest has been stimulated, firstly, by the availability of relatively cheap micro-machining techniques to enable such devices to be viable, both technically and commercially, and secondly by the realisation that many useful needs could be serviced by such devices.
Amongst these needs are those for medical applications including portable dialysis machines and intra-venous drug delivery, for instance of insulin. In the developing field of micro-fluidics, so-called lab-on-a-chip devices exploit the laminar flow characteristics of small cross-section liquid channels to perform a variety of chemical reactions, controlled mixing and liquid analysis, using very small volumes of liquids. These devices are finding increasing numbers of applications in bio-medical research. Many such de vices would benefit from the availability of a suitable and compatible micro pump either as a stand-alone or integrated component.
In the field of engineering, needs include the liquid or air cooling of microprocessors and other high power-density electronic devices, and also to the supply of ink to and around ink supplies for inkjet printers.
Pumping of air and gases is a broad field. Many applications require volumes to be pressurized, evacuated or re-circulated. Some applications require merely that air or gas be moved past a surface, for instance in cooling or drying of an object.
There are a number of ways of classifying pumps and micro pumps. Macroscopic displacement pumps have slow speeds of response, due to the inertia of the motors and spindles driving the piston or diaphragm. In applications where demand can fluctuate rapidly, or where the demand is for very low levels of pressure fluctuation, for instance in inkjet ink supplies, this leads to the need for additional apparatus to control pressure. The additional apparatus may involve the use of weirs, pressure accumulators or dampers, leading to extra complexity and costs and to lower system functionality and reliability. In addition, the swept and priming volumes of such pumps are quite large, so that for applications where only a small volume of fluid is available or affordable, such pumps are quite unsuitable.
Applications that require the movement of volumes of gases against modest backpressures are dominated by rotating fans, either axial or centrifugal in design.
Applications that require smaller volumes to be pumped against higher back pressures, for charging pressure vessels to a few atmospheres of pressure, are dominated by piston and diaphragm pumps. The same is true of applications to evacuate pressure vessels to modest vacuums. Piston and diaphragm pumps produce acoustic noise and pressure pulses in the air stream. All such pumps are slow to start up and to turn off.
Fluctuations of pressure or flow rate produced by a pump as a result of the reciprocating action of diaphragms or pistons can be problematic for some of the possible applications for which it would otherwise be suitable. For instance, in the case of inkjet ink supply systems, pressure fluctuations from the pump that appear at the nozzles in the printhead cause unwanted variations in the mass of drops ejected and in the optical density of the patterns so formed. Many applications would benefit from faster speeds of response than are available from conventional motor driven piston or diaphragm-based pumps. For instance, paint spraying requires constant pressures when spraying, but usage is intermittent, thus requiring the use of heavy and bulky pneumatic reservoirs and pumps.
Micro pumps have been largely built around reciprocating diaphragms, with valves based either on flexible flaps or fixed geometries such as nozzle-diffuser devices. Such micro-pumps are generally capable of only very limited rates of flow, of up to about 16 milliliters per minute. Such rates of flow are usually too low to be useful for some of the intended applications, for instance in many inkjet ink supplies.
Another requirement for micro-pumps is for high energy efficiency. This is important for mobile applications, particularly those where power is supplied by batteries, in order to minimise the power consumption and to maximise the time that the device can ran on the battery.
Jamming of moving parts is another potential issue. Some of the intended applications use fluids that can cause moving parts to become jammed if the system is turned off for any length of time. Examples would be the pumping of blood, insulin or ink. Pumps featuring actuators with sliding surfaces, for instance between cylinders and pistons, and valves featuring contacting surfaces, such as flap or reed valves can suffer from reliability problems due to sticking of these sub-systems. In addition, these same sliding and moving surfaces can damage the fluid being pumped. In the case of biological fluids, an example would be the rapturing of cell membranes due to excessively high shear rates or pressure. In the case of Inkjet inks, it is known that high shear rates lead to removal of surfactant chemistries from the surfaces of pigment particles, leading to clumping and precipitation of the pigment particles. In air pumps, airborne dust can prevent the pump's non-return valves from seating properly and hence can degrade the efficiency of the pump.
It would therefore be desirable to produce a pump that is physically compact and produces a flow of fluid that is both responsive to the demands of the system in terms of flow rate and also does not introduce the cyclical pressure pulses that are usually associated with positive displacement pumps.